1. Field of the Invention
This invention relates to a semiconductor integrated circuit, and more particularly to a semiconductor integrated circuit containing a circuit having an impedance circuit externally attached thereto, for example, an amplifier circuit having a load resistor provided outside a chip or a time constant circuit having a capacitor provided outside a chip.
2. Description of the Related Art
FIG. 1 shows a block construction of an integrated circuit containing an amplifier circuit AMP having a load resistor R.sub.L externally attached thereto. A portion surrounded by broken lines is an internal portion of the integrated circuit 10, and in the integrated circuit 10, a current source circuit CS for creating an operation current of the amplifier circuit AMP and a reference resistor Rref for determining the value of the current created by the current source circuit CS are provided.
In most cases, the gain G.sub.V of the amplifier circuit AMP varies in proportion to the product of the current generated by the circuit CS and the resistance of the load resistor R.sub.L, although the variation in the gain may also depend on the circuit construction. Since the current created by the current source circuit CS varies in inverse proportion to the resistance of the reference resistor Rref, the gain G.sub.V varies in proportion to the resistance of the load resistor R.sub.L and varies in inverse proportion to the resistance of the reference resistor Rref. This relation can be expressed by the following equation: EQU G.sub.V =K.multidot.(R.sub.L .multidot.Rref) (1)
where K is a constant.
In a case where the amplifier circuit AMP is a feedback amplifier circuit, the gain G.sub.V does not vary in proportion to the resistance of the load resistor R.sub.L, but the above equation (1) can be applied by taking an open loop gain into consideration.
Based on the above fact, it can be said that the gain G.sub.V varies in proportion to the resistance of a resistor provided outside the integrated circuit (chip) 10 and varies in inverse proportion to the resistance of a resistor provided inside the integrated circuit 10.
When variations in the resistances of the respective resistors are taken into consideration, the degrees of variations in the resistances of the load resistor R.sub.L and reference resistor Rref, and particularly, the degree of variation in the resistance of the reference resistor Rref in the integrated circuit 10 is larger than that in the resistance of the externally connected load resistor R.sub.L. Typically, the degree of variation in the resistance of the load resistor R.sub.L is .+-.5% and the degree of variation in the resistance of the reference resistor Rref is .+-.20%. These values can be reduced to less than 1% by using a high-precision resistor for the load resistor R.sub.L and by using a manufacturing method such as laser trimming for forming the reference resistor Rref. However, in this case, the cost will become extremely high.
Further, when the temperature characteristics of the respective resistors are taken into consideration, the temperature coefficients of the load resistor R.sub.L and the reference resistor Rref are different from each other and therefore the gain G.sub.V will be changed with a temperature variation. The load resistors R.sub.L of various temperature coefficients are available, but when it is required to use a load resistor R.sub.L of specified temperature coefficient, it may be different from a standard resistor and the cost thereof will become high. Further, since the temperature coefficient of the reference resistor Rref is determined by the manufacturing process of the circuit, it is practically impossible to change the temperature coefficient of the reference resistor Rref.
That is, with the circuit construction shown in FIG. 1, variation in the gain G.sub.V of the amplifier circuit AMP is large and the gain G.sub.V is also influenced by temperature variation so that it cannot be used in practice.
The circuit construction for solving the above problem is shown in FIG. 2. The difference between the constructions of FIGS. 1 and 2 lies in that the reference resistor Rref is provided outside the integrated circuit (chip) 10. With this construction, the problem of the circuit construction shown in FIG. 1 can be solved, but since the reference resistor Rref is externally connected, the number of external pins of the integrated circuit 10 must be increased by 2 in comparison with the case of FIG. 1. Even if one of the external pins which are connected to the reference resistor Rref is commonly used with a ground pin or the like, one pin must be additionally provided. Further, since the reference resistor Rref is externally connected, the number of externally attached parts is increased.